The application of uniform coatings to moving workpieces continues to be an important goal of spray coating systems. Although spray coating systems may use many different types of spray heads, ultrasonic spray heads have recently gained considerable attention. In general, ultrasonic spray heads function by converting a film of liquid on a vibrating surface into a mist of small drops. Various techniques for precisely controlling this ultrasonic spray head such that a uniform spray pattern is produced are disclosed in U.S. Pat. No. 5,409,163 entitled "Ultrasonic Spray Coating System with Enhanced Spray Control," which is incorporated by reference hereinabove.
One area in which ultrasonic spray coating systems may be used is the application of soldering flux to printed circuit boards ("PCBs"). As is well known, a wave soldering process is conventionally used to apply solder to a printed circuit board populated with components such that they mechanically and electrically connect to the PCB. A component's electrical lead typically extends through the PCB via a "through hole." Surrounding this through hole on either side of the PCB is a copper pad that is often coated with another conductor (e.g., silver or tin) to facilitate solderability and to protect the copper from corrosion. The interior of the through hole is also conventionally coated with a conductor. As the PCB passes through the wave solder machine its underside encounters a wave of molten solder. This solder bonds to the pads of the circuit board and the leads of the components, flowing up the through hole, thereby mechanically and electrically bonding the component to the PCB.
An applied coating of soldering flux enhances the flow of solder described above. Conventionally, the PCB encountered a flux wave prior to encountering the solder wave. This flux wave comprised a thick rosin flux which was forced up into the through holes by wave forces. Recently, rosin fluxes have been abandoned because the printed circuit boards they are used on require post-soldering cleaning using Chloro-Fluorocarbons ("CFCs") which are known to be environmentally hazardous.
As a replacement for rosin fluxes, "no-clean" fluxes have been used. These typically comprise a volatile organic compound ("VOC") used as a carrier (e.g., alcohol) and a small amount of flux (e.g., 1-5%). The carrier/flux mixture is oversprayed onto a PCB to insure complete coverage. After application of the no-clean flux, a heating of the board causes the VCO and flux to flow into the through holes such that solder flow therethrough is facilitated, and causes the VOC carrier to evaporate, leaving the flux in place. Due to the small amount of flux remaining on the board, no cleaning process is necessary after soldering. However, there are environmental concerns regarding the use of VOCs as carriers.
More recently, attempts at using water as a carrier for "no-clean" fluxes have been made with limited success. Water carried fluxes does not naturally flow across the PCB as well as VOC carried fluxes. Therefore, methods have been employed which severely over apply water carried fluxes to insure complete coverage of, and a uniform distribution over, the PCB. Further, due to the non-uniformity of conventional and ultrasonic spray nozzles, a pattern of overlapping spray is used to guarantee complete and uniform coverage (as well as to insure the over application of flux). However, even by using an over spraying technique, little flux penetrates the through holes.
An example of a conventional spraying mechanism is shown in FIG. 1. A PCB 13 moves along a conveyor 15 towards a solder wave. The PCB 13 is sprayed with a water carried flux by a conventional or ultrasonic nozzle 10 that produces a non-uniform conical spray pattern 16 when active. Nozzle 10 is guided by a rail 17 that is positioned at a right angle 12 to the direction of motion of the workpiece. During spraying, the nozzle 10 is maintained active and continuously reciprocated back and forth along rail 17. Due to right angle 12 of guide rail 17 and the motion of the PCB, a zigzag pattern of spray 14 is produced on the PCB. This spray pattern has significant overlap to compensate for the non-uniformity of the nozzle and to insure over spraying of the board with flux.
Several problems are associated with the conventional over spraying techniques for applying water carried fluxes. The excess water carrier must be evaporated prior to soldering, or poor solder joints may form due to steam being produced upon contact of unevaporated excess water with the molten solder wave. However, water does not evaporate as quickly as VOCs so a longer drying time within a heated environment is necessary for complete evaporation of the water. Furthermore, the conventional deposition of excess flux on the PCB results in a waste of flux, thereby increasing the cost of the soldering process.
Recently, the elimination of the conductive coating from the copper pads and the interior of the through holes has been identified as a cost saving measure in the PCB manufacturing process. An organic coating is applied to the copper pads and through holes, as a replacement for the conductive coating to prevent corrosion of the copper. The flux is required to break down this protective organic coating to expose the copper to the solder and facilitate soldering. With this requirement, the need for flux inside the through holes is substantially increased. However, conventional over spray methods have been unable to force adequate amounts of water carried flux into the through holes without depositing excessive amounts of flux on the bottom surface of the PCB.
The present invention is directed towards solutions to the above noted problems.